The present invention relates to a detection signal processing circuit for a rotation sensor and a related apparatus capable of detecting the rotational direction and the rotational speed of a rotor based on detection signals of two sensing elements.
The rotation sensors, capable of accurately detecting not only a rotational angle but also a rotational direction (e.g. reverse rotation such as backlash), are preferably used as crank angle sensors and cam angle sensors for automotive vehicles. FIG. 9A is a detection signal processing circuit 110 applicable to this kind of rotation sensor. The detection signal processing circuit 110 includes two magnetic sensors (not shown) disposed in a confronting relationship with a gear 50 so that two detection signals having a phase difference are produced from these two magnetic sensors. The detection signal processing circuit 110 detects the rotational direction of the gear 50 based on the detection signals of these magnetic sensors. The detection signal processing circuit 110 outputs the detected rotational direction via a signal line 144 to a rotational direction processing circuit 132 in ECU 130. On the other hand, the detection signal processing circuit 110 outputs a pulse signal representing the rotation of the gear 50 via a signal line 145 to a rotation signal processing circuit 134 of ECU 130. Furthermore, the detection signal processing circuit 110 is connected via a power source line 142 to a power source Vcc of ECU 130 and is also connected via an earth line 146 to the earth.
FIG. 9B shows a rotational direction signal and a rotational speed signal produced from the detection signal processing circuit 110. The rotational direction signal produced via the signal line 144 is 0 V when the gear 50 is rotating in the forward direction, and increases to 5 V when the gear 50 is rotating in the reverse direction. The rotational speed signal produced via the signal line 145 is a pulse signal representing the rotation of the gear 50.
The crank angle sensor and the cam angle sensor of an automotive vehicle are usually positioned in the vicinity of an engine control ECU. Using the above-described four cables (i.e. lines 142, 144, 145, and 146) shown in FIG. 9A is not so inconvenient. However, the rotation sensor detecting the rotational speed of a wheel is located far from the ECU. Using a total of four cables for each rotation sensor will cause the problems of wiring weight and wiring space.
In view of the above problems, the applicant of this invention has already proposed a detection signal processing device for a rotation sensor which is capable of reducing the number of cables (refer to the Japanese Patent Application Laid-open No. 10-332725 (1998)). According to this prior art technique, as shown in FIG. 10A, a forward/reverse judging circuit 304 detects a rotational direction of a rotor (not shown). The forward/reverse judging circuit 304 turns on a switch SW2 in response to detection of rotor's forward rotation, to supply a constant current Ib′. The forward/reverse judging circuit 304 turns on a switch SW3 in response to detection of rotor's reverse rotation, to supply a constant current Ic′. As shown in FIG. 10B, during the forward rotation, the pulse signal representing the rotation of the rotor has a current value of Ia′ at its low level and Ia′+Ib′ at its high level.
During the reverse rotation of the rotor, the pulse signal representing the rotation of the rotor has a current value of Ia′ at its low level and Ia′+Ic′ at its high level. The current Ia′ is required for activating a constant voltage circuit 302. Accordingly, the electric potential of an output line 246 is R10×(Ia′+Ib′) at the high level of the detection signal during the forward rotation of the rotor, and R10×(Ia′+Ic′) at the high level of the detection signal during the reverse rotation of the rotor. Thus, performing forward/reverse judgment with respect to the rotational direction of the rotor is feasible by using only two wiring cables of a power source line 242 and the signal line 246.
However, the above-described prior art technique requires three discriminable current values (Ia′, Ia′+Ib′, and Ia′+Ic′) for discriminating the rotational direction. Accordingly, electric power consumption of the rotation sensor will increase. As described above, when the rotation sensor is positioned far from the ECU to detect the rotation of a wheel, the electric power consumption in the wiring will also increase. This is the reason why the above-described conventional technique is not preferably applicable to heavy-duty trucks or buses due to their longer bodies.